A gas turbine is coupled, for example in a power plant for the generation of electrical energy, to a generator and is thus operated both at part load and also at full load at the same rotational speed. The gas turbine has a compressor, a combustion chamber and a turbine, wherein ambient air is drawn in and compressed by the compressor, said ambient air being heated in the combustion chamber by way of the combustion of a fuel. The heated and compressed air is expanded in the turbine with work being performed, wherein the generator is driven by means of the excess work thereby gained. The turbine is conventionally of axial type of construction, wherein the guide blade rows and rotor blade rows are arranged in an alternating fashion and one behind the other in a main flow direction. To attain the highest possible thermodynamic efficiency of the gas turbine, it is desirable for the gas turbine to be operated with as high a turbine inlet temperature as possible. The maximum admissible turbine inlet temperature is determined by the thermal load capacity of the turbine, in particular of the guide blade rows and of the rotor blade rows of the turbine.
An increase in the maximum admissible turbine inlet temperature is possible if for example the first guide blade row directly downstream of the combustion chamber outlet is cooled. For the cooling of the guide blade row, it is known for example from GB 1 338 354 and GB 938 247 for the guide blades of the guide blade row to be of hollow form and to be traversed by a flow of cooling air which is extracted for example from the compressor.
If the gas turbine is operated at part load, the turbine inlet temperature and the overall pressure ratio of the gas turbine decrease in relation to full-load operation of the gas turbine, whereby the thermodynamic efficiency of the gas turbine is disadvantageously decreased in part-load operation. Furthermore, in part-load operation of the gas turbine, the overall mass flow of the main flow through the gas turbine is reduced, whereby it is disadvantageously the case that the relative approach flow angles to the turbine rotor blades, in particular to the turbine rotor blades of the rotor blade row downstream of the first guide blade row, differ from the approach flow angles in the design state. This leads to an incorrect impingement of flow on the turbine rotor blades in part-load operation of the gas turbine, whereby the exertion of work on the turbine rotor blades is disadvantageously reduced. Furthermore, in part-load operation of the gas turbine, the fuel supply into the combustion chamber is reduced, whereby the flame temperature in the gas turbine decreases. This can result in disadvantageous combustion instabilities in the combustion chamber, which adversely affect the operation and the availability of the gas turbine in part-load operation.
If the gas turbine is switched entirely from full-load operation to part-load operation, the individual stage pressure ratios and thus the overall pressure ratio of the gas turbine decrease, and the exhaust-gas temperature increases in the case of a turbine inlet temperature remaining substantially the same. This can give rise to an operating state in which the exhaust-gas temperature exceeds a maximum admissible maximum value. An adequate reaction to this would, in said part-load operation, be to reduce the turbine inlet temperature in order that the exhaust-gas temperature is returned to a value less than or equal to its maximum admissible maximum value, this however disadvantageously being associated with a decrease in the thermodynamic efficiency of the gas turbine.
The secondary air consumption is determined, as per GB 1 338 354, by the cooling air consumption which is required at rated load. As a result of the temperature decrease at part load, it is then possible for a part of the cooling air to be saved. The saved fraction is then discharged as “jet flap air” at the trailing edge of the guide blades for the purpose of diversion manipulation. A change in the secondary air consumption as a function of the operating state is not provided in GB 1 338 354. According to GB 938 247, however, a deactivation of the cooling of the turbine guide blades may be provided.